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Journal ArticleDOI: 10.1021/JACS.1C00265

Mechanism Dictates Mechanics: A Molecular Substituent Effect in the Macroscopic Fracture of a Covalent Polymer Network.

02 Mar 2021-Journal of the American Chemical Society (American Chemical Society (ACS))-Vol. 143, Iss: 10, pp 3714-3718
Abstract: The fracture of rubbery polymer networks involves a series of molecular events, beginning with conformational changes along the polymer backbone and culminating with a chain scission reaction. Here, we report covalent polymer gels in which the macroscopic fracture "reaction" is controlled by mechanophores embedded within mechanically active network strands. We synthesized poly(ethylene glycol) (PEG) gels through the end-linking of azide-terminated tetra-arm PEG (Mn = 5 kDa) with bis-alkyne linkers. Networks were formed under identical conditions, except that the bis-alkyne was varied to include either a cis-diaryl (1) or cis-dialkyl (2) linked cyclobutane mechanophore that acts as a mechanochemical "weak link" through a force-coupled cycloreversion. A control network featuring a bis-alkyne without cyclobutane (3) was also synthesized. The networks show the same linear elasticity (G' = 23-24 kPa, 0.1-100 Hz) and equilibrium mass swelling ratios (Q = 10-11 in tetrahydrofuran), but they exhibit tearing energies that span a factor of 8 (3.4 J, 10.6, and 27.1 J·m-2 for networks with 1, 2, and 3, respectively). The difference in fracture energy is well-aligned with the force-coupled scission kinetics of the mechanophores observed in single-molecule force spectroscopy experiments, implicating local resonance stabilization of a diradical transition state in the cycloreversion of 1 as a key determinant of the relative ease with which its network is torn. The connection between macroscopic fracture and a small-molecule reaction mechanism suggests opportunities for molecular understanding and optimization of polymer network behavior.

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5 results found

Journal ArticleDOI: 10.1021/ACS.CHEMREV.0C01304
01 Apr 2021-Chemical Reviews
Abstract: Polymer networks are complex systems consisting of molecular components. Whereas the properties of the individual components are typically well understood by most chemists, translating that chemical insight into polymer networks themselves is limited by the statistical and poorly defined nature of network structures. As a result, it is challenging, if not currently impossible, to extrapolate from the molecular behavior of components to the full range of performance and properties of the entire polymer network. Polymer networks therefore present an unrealized, important, and interdisciplinary opportunity to exert molecular-level, chemical control on material macroscopic properties. A barrier to sophisticated molecular approaches to polymer networks is that the techniques for characterizing the molecular structure of networks are often unfamiliar to many scientists. Here, we present a critical overview of the current characterization techniques available to understand the relation between the molecular properties and the resulting performance and behavior of polymer networks, in the absence of added fillers. We highlight the methods available to characterize the chemistry and molecular-level properties of individual polymer strands and junctions, the gelation process by which strands form networks, the structure of the resulting network, and the dynamics and mechanics of the final material. The purpose is not to serve as a detailed manual for conducting these measurements but rather to unify the underlying principles, point out remaining challenges, and provide a concise overview by which chemists can plan characterization strategies that suit their research objectives. Because polymer networks cannot often be sufficiently characterized with a single method, strategic combinations of multiple techniques are typically required for their molecular characterization.

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14 Citations

Journal ArticleDOI: 10.1021/JACS.1C02149
Abstract: Mechanochemical reactions that lead to an increase in polymer contour length have the potential to serve as covalent synthetic mimics of the mechanical unfolding of noncovalent “stored length” doma...

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Topics: Cyclobutane (51%)

3 Citations

Journal ArticleDOI: 10.1002/ANIE.202105620
Dong Zhao1, Zhaoming Zhang1, Jun Zhao1, Kai Liu1  +6 moreInstitutions (1)
12 Jul 2021-Angewandte Chemie
Abstract: Mortise-and-tenon joints have been widely used for thousands of years in wooden architectures in virtue of their artistic and functional performance. However, imitation of similar structural and mechanical design philosophy to construct mechanically adaptive materials at the molecular level is a challenge. Herein, we report a mortise-and-tenon joint inspired mechanically interlocked network (MIN), in which the [2]rotaxane crosslink not only mimics the joint in structure, but also reproduces its function in modifying mechanical properties of the MIN. Benefiting from the hierarchical energy dissipative ability along with the controllable intramolecular movement of the mechanically interlocked crosslink, the resultant MIN simultaneously exhibits notable mechanical adaptivity and structural stability in a single system, as manifested by decent stiffness, strength, toughness, and deformation recovery capacity.

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Topics: Mortise and tenon (51%)

1 Citations

Journal ArticleDOI: 10.1021/JACS.1C05857
Matias Horst1, Jinghui Yang1, Jan Meisner1, Tatiana B. Kouznetsova2  +4 moreInstitutions (3)
Abstract: We have recently reported a series of ladder-type cyclobutane mechanophores, polymers of which can transform from nonconjugated structures to conjugated structures and change many properties at once. These multicyclic mechanophores, namely, exo-ladderane/ene, endo-benzoladderene, and exo-bicyclohexene-peri-naphthalene, have different ring structures fused to the first cyclobutane, significantly different free energy changes for ring-opening, and different stereochemistry. To better understand their mechanochemistry, we used single molecule force spectroscopy (SMFS) to characterize their force-extension behavior and measure the threshold forces. The threshold forces correlate with the activation energy of the first bond, but not with the strain of the fused rings distal to the polymer main chain, suggesting that the activation of these ladder-type mechanophores occurs with similar early transition states, which is supported by force-modified potential energy surface calculations. We further determined the stereochemistry of the mechanically generated dienes and observed significant and variable contour length elongation for these mechanophores both experimentally and computationally. The fundamental understanding of ladder-type mechanophores will facilitate future design of multicyclic mechanophores with amplified force-response and their applications as mechanically responsive materials.

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Journal ArticleDOI: 10.1246/CL.210307
Jingyuan Nie1, Fang Tian1, Bin Zheng1, Ziyi Wang1  +1 moreInstitutions (1)
19 Jun 2021-Chemistry Letters
Abstract: Thanks to the binding of various metal ions, metalloprotein plays an essential role in many different biological processes and represents an indispensable protein subgroup. Thus, the knowledge of t...

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Topics: Ligand (59%), Molecule (56%), Force spectroscopy (53%)

41 results found

Journal ArticleDOI: 10.1002/POL.1953.120100303
R. S. Rivlin1, A. G. Thomas1Institutions (1)
Abstract: The resistance to tearing of a rubber vulcanizate is usually determined by loading in a specified manner a test-piece of the vulcanizate of standard shape, in which a notch has been produced, either in the molding process or by cutting the test-piece in a standard fashion. A wide variety of shapes of test-piece and notch and of methods of loading have been recommended by various authors (see, for example, Buist1).

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Topics: Tearing (56%), Natural rubber (56%)

1,010 Citations

Journal ArticleDOI: 10.1098/RSPA.1967.0160
G. J. Lake1, A. G. Thomas1Institutions (1)
Abstract: Under repeated stressing, cracks in a specimen of vulcanized rubber may propagate and lead to failure. It has been found, however, that below a critical severity of strain no propagation occurs in the absence of chemical corrosion. This severity defines a fatigue limit for repeated stressing below which the life can be virtually indefinite. It can be expressed as the energy per unit area required to produce new surface ( T 0 ), and is about 5 x 10 4 erg/cm 2 . In contrast with gross strength properties such as tear and tensile strength, T 0 does not correlate with the viscoelastic behaviour of the material and varies only relatively slightly with chemical structure. It is shown that T 0 can be calculated approximately by considering the energy required to rupture the polymer chains lying across the path of the crack. This energy is calculated from the strengths of the chemical bonds, secondary forces being ignored. Theory and experiment agree within a factor of 2. Reasons why T 0 and the gross strength properties are influenced by different aspects of the structure of the material are discussed.

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584 Citations

Open accessJournal ArticleDOI: 10.1126/SCIENCE.1248494
11 Apr 2014-Science
Abstract: Elastomers are widely used because of their large-strain reversible deformability. Most unfilled elastomers suffer from a poor mechanical strength, which limits their use. Using sacrificial bonds, we show how brittle, unfilled elastomers can be strongly reinforced in stiffness and toughness (up to 4 megapascals and 9 kilojoules per square meter) by introducing a variable proportion of isotropically prestretched chains that can break and dissipate energy before the material fails. Chemoluminescent cross-linking molecules, which emit light as they break, map in real time where and when many of these internal bonds break ahead of a propagating crack. The simple methodology that we use to introduce sacrificial bonds, combined with the mapping of where bonds break, has the potential to stimulate the development of new classes of unfilled tough elastomers and better molecular models of the fracture of soft materials.

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554 Citations

Journal ArticleDOI: 10.1039/B603438A
Abstract: New PEG-based hydrogel materials have been synthesized by Click chemistry and shown to result in well-defined networks having significantly improved mechanical properties; the selectivity of the azide/acetylene coupling reaction also allows for the incorporation of various additives and functional groups leading to chemical tailoring of the hydrogels.

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Topics: Click chemistry (61%), Self-healing hydrogels (60%), Azide (53%)

518 Citations

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